1. Technical Field
The disclosure relates generally to the field of marine geophysical surveying. More particularly, this disclosure relates to systems and methods for reducing drag on cables used in marine geophysical surveying.
2. Description of the Related Art
Marine geophysical surveying systems, such as seismic acquisition systems and electromagnetic survey systems, are used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or the ocean. One type of survey, a marine seismic survey, is based on the use of sound waves. In such a survey, a vessel may tow one or more acoustic sources (e.g., an air gun) and a plurality of streamers along which a number of sensors (e.g., hydrophones) are located. Sound waves generated by the source may then be transmitted to the earth's crust and then reflected back by various strata and detected at the sensors. Sound waves received during a marine seismic survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located. In a similar fashion, marine electromagnetic (EM) surveys may be conducted using EM signals transmitted by, for example, a submerged antenna and detected by EM receivers.
In the example of marine seismic surveying, the surveying systems typically include a seismic survey vessel having onboard navigation, seismic signal source control, and geophysical data recording equipment. The seismic survey vessel is typically configured to tow one or more (typically a plurality of) laterally spaced sensor streamers through the water. At selected times, the seismic signal source control equipment causes one or more seismic signal sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy are ultimately conducted to the recording equipment. A record is made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals may be interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may also be used in marine electromagnetic geophysical survey systems.
In some embodiments, it may be advantageous for the geophysical survey sources to be towed at relatively wide lateral separations (e.g. relatively wide separations from other survey sources, or relatively wide separations from a central axis of the survey vessel). Certain marine geophysical surveying systems and a discussion of some advantages of wide source tow are provided in U.S. Patent Application Publication No. 2012/0257474, which is incorporated by reference herein. Reducing hydrodynamic drag on sources and source cables may be advantageous in attaining relatively wide tow of such sources. Further, even in embodiments that do not involve wide tow of sources (e.g., sources directly behind a survey vessel, or sources at relatively small lateral separations from a central axis thereof), it may be desirable to reduce the hydrodynamic drag acting on the sources and/or source cables.